The invention is directed to a position signaling device, especially for industrial sewing machines, having a rotatably mounted slotted wheel which can be coupled with a drive shaft of the machine and which has a plurality of equally wide and equidistantly disposed slots and lands forming a first circular track. The device also has fixed slotted masks disposed in back of the slotted wheel in the axial direction, offset from one another circumferentially by a defined angle, and disposed coaxially with the slotted wheel, comprising slots and lands corresponding to the division of the slotted wheel. Each fixed mask has a light emitter or a light receiver disposed axially parallel thereto behind each slotted mask, and having each a light receiver or light emitter, respectively, disposed axially parallel on the other side of the slotted wheel, an electronic evaluating system for determining the rotatory speed and the angle of rotation according to the number of counted light pulses caused by the slots, and for determining the direction of rotation according to the phase shift of the signals on the basis of the angular offset of the slotted masks.
A position indicating device of this kind is disclosed in U.S. Pat. No. 3,723,748, which is hereby incorporated by reference.
For many applications of position indicating devices, e.g., in machine tools, it is necessary that the absolute angular position of certain angular positions of the drive shaft be detected, since these positions correspond to corresponding positions of the tool. In sewing machines, for example, there are two common stop positions, which are designated as position 1 and position 2 and correspond to the needle-raised position and the needle lowered position, respectively. These positions are defined in reference to a null point of the position indicating device. After the machine is turned on, a null-point synchronization must be performed. Conventional position signaling devices must for this purpose cover at least a range of angles of 180.degree. and 360.degree., respectively, in order to permit the null point to be known with certainty. For the establishment of the definite angular positions referred to above, e.g., position 1 and position 2, in addition to the slotted wheel for measuring the rotatory speed, additional wheels are conventionally needed coaxially with the slotted wheel and at a distance from the latter, e.g., a second wheel for establishing stop position 1 and a third wheel to establish stop position 2.